The Bayer process has been used to recover alumina values from bauxitic ores for over a century. The process centres on the following reversible equations, for gibbsitic and boehmitic or diasporic ores, respectively (1):Al(OH)3+OH−Al(OH)4−  (1)AlO(OH)+OH−+H2OAl(OH)4−  (2)
A schematic flowsheet showing a basic implementation of a traditional Bayer Process is illustrated in FIG. 1. Blended bauxite ore is first mixed with a portion of the recycled spent liquor and subjected to grinding to reduce particle size. The resultant slurry is then treated via a process known as “desilication” or “slurry holding” to remove soluble silica minerals present in the bauxite, typically in the form of insoluble sodium aluminosilicates.
The desilicated slurry is then mixed with the remainder of the spent liquor and the alumina values of the bauxite extracted via a process referred to as “digestion”. In digestion, the conditions are manipulated so as to drive equation (1) or (2) towards the right hand side. During digestion, the free caustic dissolves the aluminous mineral from the bauxite to form a concentrated sodium aluminate solution leaving behind a mud residue of undissolved minerals and impurities, principally inert iron oxides and hydroxides, titanium oxides and silicious compounds. The mud residue is often red in appearance due to the presence of the iron minerals and is thus commonly referred to as “red mud”. Digestion is favoured by using conditions of high temperature and pressure and these are in turn dependent on the type of ore being treated. Gibbsitic bauxite can be digested at temperatures ranging from about 100-180° C., although 145° C. is most common. Boehmitic or diasporic bauxites are less soluble and require temperatures in the vicinity of 250-270° C. to effect digestion. The equilibrium expressed in equations (1) and (2) can also be displaced to the right hand side by increasing the concentration of free caustic (hydroxyl ions).
In a typical alumina refinery, steam is used to heat the desilicated slurry to the temperature required for digestion. This steam is partially recovered from a series of flash coolers used to reduce the temperature and/or pressure of the mud-laden pregnant liquor that leaves the digesters. The final stage of heating uses high pressure steam from a boiler, usually a powerhouse boiler. Typically the flash coolers are used to reduce the temperature of the mud laden pregnant liquor to the atmospheric boiling point by flowing through a series of flash vessels which operate at successively lower pressures.
After flash cooling, the pregnant liquor is separated from the mud residue in a process referred to as “clarification”. The slurry is fed to one or more settling tanks in which the solid particles sink to the bottom and are removed, typically by pumping to the mud washing circuit. Flocculants may be added to the settling tanks to improve the rate of mud settling and achieve good clarity in the settler overflow liquor.
The mud washing circuit relies on a counter-current decantation process to recover as much sodium aluminate as possible for re-use to minimise loss of alumina and caustic values and to cleanse the mud residue so that it can be disposed of in an environmentally acceptable manner. The washer overflow that subsequently exits the first stage mud washing tank is either directed to the settling tanks, or mixed with the settler overflow liquor to form clarified pregnant liquor. The washed mud residue from the final stage in the mud washing circuit is typically pumped to a mud disposal lake. The counter-current mud washing circuit is fed with wash water, typically fresh water, condensate (condensed steam) or recycled water from the mud disposal lake (known as “lake water”), or combinations of the above.
The clarified pregnant liquor which overflows the settling tanks is subjected to filtration before being sent to the precipitation stage in which the equilibrium of equation (1) (reproduced again below) is driven towards the left hand side to form pure Al(OH)3, also referred to as “gibbsite”.Al(OH)3+OH−Al(OH)4−  (1)
Precipitation is initiated by seeding, and is favoured by conditions that increase the supersaturation of the liquor, such as reducing the temperature, increasing the concentration of aluminate ions, or diluting the solution. The precipitated gibbsite is separated via hydrocyclones, thickeners or filters. The remaining liquor, after evaporation to remove excess water that has entered the process with the bauxite and various washing steps is referred to as “spent liquor” and will have aluminate ions and hydroxyl ions present in an amount that depends on the temperature, seed surface area and residence time of the precipitation stage. To recover the alumina values and caustic, the spent liquor is recycled to digestion. Thus the spent liquor that is recycled to digestion has dissolved alumina present in it.
The primary goal of the Bayer process is to extract the maximum amount of alumina values (Al) from the bauxite fed to digestion into solution and then completely recover this dissolved alumina from the solution in the form of gibbsite during precipitation. The upper limit of the refinery's precipitation yield is set by the difference between the solubility of alumina in a particular liquor at the digestion temperature and the solubility of alumina in that liquor at the temperature used for precipitation. It follows then, that maximizing this difference is a primary aim of most alumina refineries.
Increasing the solubility of an aluminous mineral by raising the temperature or caustic concentration carries a number of unwanted consequences, including increasing the dissolution of siliceous materials. In the prior art, one method for increasing the alumina content of Bayer liquors derived from the less soluble boehmitic and diasporic ores involves “sweetening” the liquor (after digestion of the primary ore) with a small amount of a secondary gibbsitic ore, in a second digestion step.
To date, there is no known method of increasing the alumina concentration of the liquor after digestion.
One of the main avenues for alumina loss in an alumina refinery is in the liquor that is pumped with the mud residue from the settling tanks into the mud washing circuit. This liquor is supersaturated pregnant liquor having effectively the same concentration of aluminate ions as the pregnant liquor sent to precipitation. The liquor that overflows each stage in the counter-current mud washing circuit becomes progressively cooler and more diluted with wash water. This effectively increases the supersaturation of the liquor, encouraging precipitation of gibbsite in accordance with equation (1). The mud particles in the residue have a high surface area that further encourages such precipitation of gibbsite in the mud washing circuit. Any alumina that precipitates in the mud washing circuit in this manner is lost, as is any dissolved alumina in the liquor reporting to the mud disposal lake.
There is a need for an alternative method of maximizing the recovery of alumina values in an alumina refinery.